Bottle caps for carbonated beverages have for many years been constructed of metal. These caps are either crimped to the neck of the bottle or threaded onto the bottle. More recently the industry has turned to the use of plastic caps constructed either for a snap fit onto the bottle or threaded engagement with cooperating threads on the neck of the bottle. Plastic caps of the threaded construction are either designed for use with glass bottles or plastic bottles. The former use requires internal sealing configurations which can accommodate imperfections usually found in the top of the neck of glass bottles as resulting during their manufacturing or caused by rough handling. A typical plastic cap design for glass bottles is disclosed in U.S. Pat. No. 4,090,631. Although specifically constructed for glass bottles, these caps can also be used for plastic bottles. However, their construction is of such intricate design that the molding dies have to be correspondingly intricate. This in turn makes the molds subject to damage of the delicate parts during handling.
In constructing plastic caps as opposed to metal caps, more attention has to be given to the internal sealing flanges found on these caps so as to provide a suitable sealing of the contents in the bottle. This is more difficult with plastic caps because of their inherent physical characteristics which tend to permit the sealing effect to be lost during handling and storage of the bottles. In the use of plastic caps, there has also been encountered what is called the projectile effect which is caused by the cap providing too good a seal with the bottle neck. Where this occurs, the pressure within the bottle is not released until the cap is almost completely unthreaded. At this time, the internal pressure, instead of being released gradually, is released just at the time the cap is completely unthreaded and this causes the cap to be blown off the bottle with some force. This is particularly a problem where plastic caps are used with plastic bottles because plastic bottles do not have the usual inperfections that glass bottles have.
Although the imperfections in a glass bottle may promote problems as far as obtaining a good seal, these same imperfections help to eliminate the projectile effect since the pressure within the bottle can usually be released while the cap is still adequately threaded onto the bottle. The precision molding of plastic bottles does not produce these imperfections and thus the seal of the cap tends to be maintained until just before the cap is fully removed.
U.S. Pat. Nos. 3,441,161 and 4,143,785 disclose caps constructed of plastic and adapted to be attached to bottles either by snap fit or threaded connection. These patents, however, disclose no provisions for assuring harmless release of the pressure built up in the bottle. The bottle cap disclosed in U.S. Pat. No. 4,090,631 does disclose a cap which is constructed to release the pressure within the bottle before reaching the state where the cap would be subjected to being blown off by the internal pressure. As mentioned, however, this cap is of intricate construction as is the mold from which it is formed.
The caps disclosed in these patents also rely mainly on face-to-face contact of the cap with the opposed surfaces of the bottle for maintaining a seal. With plastic caps, simple surface contact does not at all times provide a seal which is adequate. This is mainly due to the absence of a resiliency of the seal at the cap bottle sealing interface.